This invention relates to apparatus for controlling the position of an arm of a record player relative to a record disc and, more particularly, to such apparatus whereby a transducer which is provided at one end of the arm is enabled to scan a predetermined trace, such as a spiral groove, along the record disc regardless of deformations in the record disc, such as a warped surface.
In record players, such as in a phonograph record player, an optical record disc player, and the like, it is assumed that the surface of the record disc upon which prerecorded information is played back is flat. Thus, a playback transducer, such as a stylus which scans a spiral groove of the record disc, or an electrostatic or optical pickup device which scans spiral tracks, can have a relatively light force exerted thereon, thereby providing favorable playback characteristics. For example, in a phonograph record player, the force exerted on the stylus can be less than five grams, and preferably less than two grams, whereby vibrations in the stylus are due solely to undulations in the scanned groove. These stylus vibrations are, of course, translated into electrical signals which, in turn, are reproduced as high fidelity audio sounds. It is generally accepted that audio information is reproduced with greater fidelity if a lesser force is exerted on the stylus. Since the stylus generally is mounted at one end of a tone arm, the tone arm usually is counterbalanced so as to minimize the force exerted thereby on the stylus.
Unfortunately, although most phonograph record discs are manufactured with a relatively flat surface, it is quite common for a record disc to warp over a period of time. This deformation may arise due to the manner of storing, shipping or displaying phonograph record discs, and also may be due to various unfavorable conditions in the manner in which the user stores or maintains the record disc. In any event, if the record disc warps, the resultant deformations in the surface thereof exert vertical forces on the stylus when the warped record disc is played. These vertical forces have the effect of opposing the relatively light force exerted on the stylus by the tone arm, thereby resulting in the stylus skipping over a groove. Thus, the sound which is reproduced from the warped record disc by such a stylus is distorted, and is less than satisfactory.
One proposal for avoiding the skipping of a groove by the stylus in a record player due to surface deformations of a warped record disc is described in U.S. Pat. No. 3,623,734. Although this proposal generally is effective to prevent the stylus from skipping over a groove of a warped record disc, the apparatus described in this patent results in phase modulation distortion of the reproduced audio sounds. This phase modulation distortion is caused by movement of the stylus relative to the record disc in a radical direction. That is, since a surface deformation of the record disc appears as a vertical rise, the stylus which scans the record disc likewise will be moved in the vertical direction. Of course, the stylus usually is mounted in a cartridge which, in turn, is supported at one end of the pivotal tone arm. Hence, vertical movement of the stylus is translated into an angular movement of the tone arm about a pivot axis. Now, since the length of the tone arm is fixed, vertical movement of the stylus is accomplished by a radial, outward movement. The geometry which results in this radial movement can be seen from FIG. 1. Thus, when the stylus moves in the vertical direction, it also moves in the radial, or horizontal direction, with respect to the record disc. This horizontal movement, or horizontal velocity, of the stylus varies because of the surface deformations of the warped record disc. Such varying horizontal velocity produces a phase modulation distortion in the reproduced audio signals, resulting in a loss of fidelity of the audio sounds.
Another defect accompanying the proposal described in the above-mentioned patent specification is the introduction of crosstalk between left and right channels which are recorded in the same groove of a stereo record disc. Since the stylus is relatively fixed with respect to the tone arm, a pivotal movement, or angular displacement of the tone arm changes the vertical tracking angle of the stylus. That is, when the tone arm is in its normal, horizontal position, the stylus assumes its normal, vertical tracking angle with respect to the record groove. However, as the stylus is moved in the vertical direction, the tone arm pivots so as to change the vertical tracking angle of the stylus with respect to the groove. Consequently, whereas separate left and right audio channels which are recorded in the same groove are reproduced without crosstalk therebetween when the tone arm is in its proper, horizontal position, the change in the vertical tracking angle of the stylus caused by angular displacement of the tone arm introduces undesired crosstalk between these left and right audio channels.
Therefore, although the prior art provides apparatus wherein the stylus of a tone arm does not skip over a groove in a warped record, this apparatus is accompanied by undesired phase modulation distortion and crosstalk in the signals which are reproduced from the record groove by the stylus.